Lesson 39: kill Command

The kill command sends a signal — a specified signal — to a currently running process. The kill command can be executed in several ways, directly or from a shell script.

Using the kill command from /usr/bin provides you with some extra features to kill a process by process name using pkill.

kill Command Syntax

The common syntax for a kill command is:

# kill [SIGNAL or OPTION] PID(s)

kill Command Signals

Clearly, from the behavior above, SIGTERM is the default and safest way to kill a process.

SIGHUP is a less secure way of killing a process than SIGTERM. SIGKILL is the most unsafe way among the above three — it terminates a process without saving.

In order to kill a process, we need to know the Process ID of the process. A process is an instance of a program.

Every time a program starts, a unique PID is automatically generated for that process.

Every process in Linux has a PID. The first process that starts when a Linux system is booted is the init process, it is assigned a value of "1" in most cases.

init is the master process and cannot be killed this way, which ensures that the master process doesn't get killed accidentally.

init decides and allows itself to be killed, where kill is merely a request for a shutdown.

1. List All Running Linux Processes

To know all the processes and correspondingly their assigned PID, run the following ps command.

# ps -A

  PID TTY          TIME CMD
    1 ?        00:00:42 systemd
    2 ?        00:00:00 kthreadd
    3 ?        00:00:00 rcu_gp
    4 ?        00:00:00 rcu_par_gp
    5 ?        00:00:00 slub_flushwq
    6 ?        00:00:00 netns
    8 ?        00:00:00 kworker/0:0H-events_highpri
   10 ?        00:00:00 mm_percpu_wq
   11 ?        00:00:00 rcu_tasks_kthread
   12 ?        00:00:00 rcu_tasks_rude_kthread
   13 ?        00:00:00 rcu_tasks_trace_kthread
   14 ?        00:00:03 ksoftirqd/0
   15 ?        00:01:09 rcu_preempt
   16 ?        00:00:03 migration/0
   17 ?        00:00:00 idle_inject/0
   19 ?        00:00:00 cpuhp/0
   20 ?        00:00:00 cpuhp/1
   21 ?        00:00:00 idle_inject/1

You can also customize the above output using the pidof command to find the PID of a specific process.

# pidof mysqld

1684

Another way to achieve the above goal is to use the following syntax:

# ps aux | grep mysqld

root      1582  0.0  0.0   5116  1408 ?        S    09:49   0:00 /bin/sh /usr/bin/mysqld_safe --datadir=/var/lib/mysql --socket=/var/lib/mysql/mysql.sock --pid-file=/var/run/mysqld/mysqld.pid --basedir=/usr --user=mysql
mysql     1684  0.1  0.5 136884 21844 ?        Sl   09:49   1:09 /usr/libexec/mysqld --basedir=/usr --datadir=/var/lib/mysql --user=mysql --log-error=/var/log/mysqld.log --pid-file=/var/run/mysqld/mysqld.pid --socket=/var/lib/mysql/mysql.sock
root     20844  0.0  0.0   4356   740 pts/0    S+   21:39   0:00 grep mysqld

2. Kill a Process by PID